"CBC with diff" means complete blood count with differential, which is a panel of blood tests used to assess a person's health and detect many diseases, according to WebMD. "Differential" refers to a laboratory test that provides the type and count of different white blood cells.
A complete blood count provides information on the blood concentration of red blood cells, platelets, hematocrit and hemoglobin, explains WebMD. A white blood cells differential includes the count of neutrophils, lymphocytes, monocytes, eosinophils and basophils, which provides health care providers with important information about immune system function. An increase in white blood cells in the bloodstream can be an indication of infection.